Disk drives are well known in the computer art for providing secondary mass storage with random access. A disk drive comprises one or more magnetic data storage disks rotated on a spindle by a spindle motor within an enclosed housing. A magnetic read/write head (transducer or slider) with an air bearing surface is placed on an actuator and positioned very closely to a corresponding disk surface.
Disk drives currently achieve state of the art magnetic recording using heads with magneto-resistive (MR) read elements. MR heads operate with an average fly height (head-disk clearance) of about 15 nm (0.6 microinch), however manufacturing and environmental variability cause fly height variability which leads to head-disk contact at a small percentage of the head-disk interfaces. Head-disk contact results in friction forces and thermal transients which cause problems with magnetic recording. Disk drives in the future shall require fly heights to be reduced to 10 nm or below to achieve the required signal-to-noise ratios at increased bit densities and track densities. This will increase the percentage of head-disk interfaces which experience contact.
High fly write (high fly height during a write operation) occurs when the head attempts to write to the disk and the magnetic clearance between the head and the disk is much higher than normal. High fly write can be due to particle contamination, particle interaction, disk lubricant with non-uniform distribution (at the micron or sub-micron scale), disk lubricant or debris pickup, and disk asperities (protrusions). High fly write can lead to missing or poorly written information on the disk.
The close proximity of the head to the disk enables very high-resolution servo patterns and user data to be recorded on the disk. The servo patterns are typically written in servo sectors which are interleaved between data sectors or blocks. The servo patterns provide a servo controller with head position information to enable a head positioner, such as a rotary voice coil motor, to move the actuator and therefore the head from track-to-track during random access track seeking operations, and to maintain the head in proper alignment with a track during track following operations when user data is written to or read from the available data sectors.
The servo controller receives head position readings from the head and determines the head position from the servo patterns. The servo patterns may include the track number and indicate how far the head is from the track centerline. As the head passes over the servo patterns, the track identification and position indicators are read by the head and supplied to the servo controller.
Servo track writing in which the servo patterns are written to the disk is becoming an increasingly difficult and expensive part of the manufacturing process. A servo track writer is typically stabilized on a large granite base to minimize unwanted vibration and employs laser interferometry for precise position measurements. The servo track writer supplies power to the spindle motor for rotating the disk and may include a fixed head for writing a clock track to one disk surface. The servo track writer typically requires direct mechanical access to the actuator to move the actuator and the head very precisely across the disk as the head writes the track address and the servo patterns at servo sectors for each track.
Servo track writing is degraded by two major factors that perturb the fly height or fly stability of the head which causes the servo patterns to be written with deviations from ideal, circular, evenly spaced servo patterns. The first factor is external vibration which leads to non-circular tracks (non-repeatable runout) and variable track squeeze. The second factor is head-disk interaction which leads to high fly write. During servo track writing, high fly write can cause missing, degraded or improperly written servo patterns which can range from a single track to a wide annular region on the disk.
Head-disk interaction is more likely to occur during the initial operation of the disk drive. Once the disk drive has operated for an extended period of time, particles tend to get swept out of the flyable zone and the disk lubricant tends to get smoothed out. In addition, small asperities on the head and the disk tend to get burnished out in the first minutes to hours of disk drive operation. However, the most critical disk drive write operation is servo track writing where a blank disk that has been stored in a cassette for an extended period of time is loaded into the disk drive and flown on for the first time since leaving the media factory. Lack of a break-in period for the head-disk interface before servo track writing may result in head-disk interactions during servo track writing that cause non-circular tracks, variable track squeeze and missing or degraded servo bursts. In addition, since servo track writing includes little or no reading from the disk, the fidelity of the servo patterns is determined later during self-test of the disk drive.
Conventionally, a burnishing head removes asperities or contamination from the disk that may interfere with the head in order to precondition the disk before servo track writing. An extended time period is spent sweeping the burnishing head across the disk in either a linear or butterfly fashion. However, long preconditioning sweeps are time consuming and short preconditioning sweeps are ineffective because the head flies at the normal fly height.
There is, therefore, a need for reducing the time for preconditioning the head-disk interface before servo track writing.